This invention relates to electrostatographic imaging systems and more specifically to an improved ferrite toner carrier core composition which may be made from fly ash derived magnetite.
The development of electrophotographic images involves either the use of liquid toner, a single component "magnetic" toner, or a dual component system using dried toner mixed with a coated toner carrier core. The majority of today's copiers and printers use dual component systems.
When two dissimilar materials are mixed, each receives a static electric charge of equal and opposite polarity. The toner carrier part of the dual component carrier/toner system serves both the purpose of triboelectrically charging the toner and transporting it to the photoreceptor. An advantage of the invention is that the toner can be uniformly charged to one polarity, with alteration in the sign of the charge or its extent possible by changing the chemistry of either toner or carrier. Additionally, the carrier scavenges unwanted toner that otherwise would contribute to a loss in quality through background. To function properly, the carrier must uniformly charge and transport the toner, be non-abrasive to the photoreceptor and have long life. Properties of the carrier which control this behavior include particle size and shape, specific gravity, mass flow, magnetic behavior and wetting and adhesion behavior with respect to carrier coatings if used.
Electric utilities consume hundreds of millions of tons of coal annually in the United States. Due to ever increasing costs of petroleum-based fossil fuels and a national energy policy of reducing dependence on foreign-source fuel such as oil by shifting to coal, electric utilities are now projected to use nearly 800 million tons of coal annually by 1985. Fly ash, the predominant residue of coal burning, has in the past presented disposal problems to users of significant amount of coal. Despite the national focus on resource recovery and recycling during the recent past decades, and the doubling of the percentage of fly ash utilized over the period 1966 to 1978, the year 1978 saw the collection of over 48 million tons of fly ash by electric utilities alone and utilization of only 8 million tons of that total. An estimated 68 million tons of fly ash are annually produced in the United States and the significant and perhaps even dramatic anticipated shift from petroleum to coal in fossil fuel generating stations can be reasonably expected to greatly increase the amount of fly ash collected in the future.
Although to date many coal-fired generating stations have been located near sources of coal where ash disposal problems may be presumed to be minimal, as oil-fired units far removed from coal fields convert to coal under the contemporary pressures of economics and national policies, ash disposal can be expected to develop into an ever-increasing problem which, when coupled with increasingly stringent federal, state and local regulation of landfills, water quality and waste disposal generally, will present significant challenge and expense to such large scale coal users. Of the 8 million tons of fly ash utilized in 1978, almost one-third of that was utilized from disposal sites, i.e. after the producers of the fly ash had already incurred disposal costs.
Of the approximately 8 million tons of fly ash utilized in 1978, about two-thirds of it was used commercially in such applications as concrete products, cement, fill and the like.
In the instant invention raw fly ash is processed to obtain magnetite suitable for use in electrostatic imaging systems.
It is well known to form and develop images on the surface of photoconductive materials by electrostatic methods such as described, for example, in U.S. Pat. Nos. 2,297,691; 2,277,013; 2,551,582; 3,220,324; and 3,220,833. In summary, these processes as described in the aforementioned patents involve the formation of an electrostatic latent charged image on an insulating electrophotographic element and rendering the latent image visible by a development step whereby the charged surface of the photoconductive element is brought into contact with a developer mixture. As described in U.S. Pat. No. 2,297,691, for example, the resulting electrostatic latent image is developed by depositing thereon a finely-divided electroscopic material referred to in the art as toner, the toner being generally attracted to the areas of the layer which retain a charge thus forming a toner image corresponding to the electrostatic latent image. Subsequently, the toner image can be transferred to a support surface such as paper and this transferred image can be permanently affixed to the support surface using a variety of techniques including pressure fixing, heat fixing, solvent fixing, and the like.
Many methods are known for applying the electroscopic particles to the latent image including cascade development, touchdown and magnetic brush as illustrated in U.S. Pat. Nos. 2,618,552; 2,895,847 and 3,245,823. One of the most widely used methods is cascade development wherein the developer material comprising relatively large carrier particles having finely-divided toner particles electrostatically clinging to the surface of the carrier particles is conveyed to and rolled or cascaded across the electrostatic latent image-bearing surface. Magnetic brush development is also known and involved the use of a developer material comprising toner and magnetic carrier particles which are carried by a magnet so that the magnetic field produced by the magnet causes alignment of the magnetic carriers in a brush-like configuration. Subsequently, this brush is brought into contact with the electrostatic latent imagebearing surface causing the toner particles to be attracted from the brush to the electrostatic latent image by electrostatic attraction, as more specifically disclosed in U.S. Pat. No. 2,874,063.
Carrier materials used in the development of electrostatic latent images are described in many patents including, for example, U.S. Pat. No. 3,590,000. The type of carrier material to be used depends on many factors such as the type of development used, the quality of the development desired, the type of photoconductive material employed and the like. Generally, however, the materials used as carrier surfaces or carrier particles or the coating thereon should have a triboelectric value commensurate with the triboelectric value of the toner in order to generate electrostatic adhesion of the toner to the carrier. Carriers should be selected that are not brittle so as to cause flaking of the surface or particle break-up under the forces exerted on the carrier during recycle as such causes undesirable effects and could, for example be transferred to the copy surface thereby reducing the quality of the final image.
U.S. Pat. No. 4,319,988 discloses a process for separation of magnetite from raw fly ash. The magnetite is then used in the cleaning of coal. Magnetite derived from fly ash is basically spherical in nature with the exception of clusters of two or more particles. The object of the invention is to provide a material composed of free spherical particles of the proper density, size, and flowability, and essentially free of material which is not suitably attracted by magnetic forces. This is accomplished by sizing and magnetic and physical separation steps to remove substantially all unsuitable particles. Grinding may be employed in the liberation and separation of clusters.
Numerous patents suggest the use of magnetite in a toner carrier. For example, see U.S. Pat. Nos. 3,565,805, 3,795,618, 2,898,170, 4,076,640, 4,233,387, 4,265,995, 4,287,287 and 4,301,228. In U.S. Pat. No. 4,076,640 a process is described for manufacturing spherizoidized particles having an average saturation magnetic moment of between 50 and 85, and preferably at least 70 electromagnetic units per gram. However, the prior art does not teach or suggest a ferrite toner carrier composition derived from fly ash or a process for making it.